This invention relates to formation of electrodes on nonconductive substrates, and, more particularly, to the preparation of nonmagnetic electrodes on quartz glass.
In certain types of apparatus, it is necessary to have a highly conductive metallic layer or electrode on a nonconductive substrate. The most challenging aspect of preparing such metallic coatings is achieving good adherence or sticking of the metal to the substrate. If adherence is not attained, the metal layer peels or flakes away from the substrate during use. The deposition of conductive layers on quartz glass is of sufficient commercial importance that a technique for preparing such structures has been developed. A conductive silver electrode can be vapor deposited or sputtered onto a glass substrate, after first depositing a thin layer of chromium to promote adherence between the metallic coating and the glass. The resulting electrode is of high quality and is strongly adherent, but is also magnetic.
It has long been known that the silver cannot be deposited directly onto the quartz glass, because the silver does not adhere to the quartz glass in the absence of the chromium "sticky metal" layer. In many applications, the requirement of the initial chromium layer poses no problem. However, chromium is a magnetic material, and its use in some applications is unacceptable. No nonmagnetic substitute for the chromium is known for use in this application. Consequently, there is no technique for depositing fully nonmagnetic, highly conductive electrodes onto quartz glass substrates.
Fully nonmagnetic electrodes are required in certain applications, such as a spaceborne hydrogen maser atomic clock. In such a clock, a quartz glass bottle containing atomic hydrogen is placed into a resonant microwave cavity, wherein the microwaves can excite the hydrogen atoms in the bottle to resonate the cavity. The bottle must have conductive electrodes on the outer surface of the bottle, and electrodes must be fully nonmagnetic to avoid adverse magnetic reactions with the microwave excitation that might cause frequency shifting of the maser radiation. The electrodes must be highly conductive, of either copper or silver. Additionally, the electrodes must be firmly and integrally affixed to the quartz bottle in order to achieve the thermal stability of the quartz, as the metal electrodes themselves have many times the thermal expansion coefficient or quartz. The known techniques for depositing these electrode materials onto the glass surface do not result in acceptable electrodes, either because the electrode is not sufficiently adherent to its substrate or because the deposition process results in a magnetic material in the deposited structure, such as the chromium undercoating described previously.
One alternative is to fix thin sheets of the electrode material to the quartz glass substrate by physical means. For example, copper sheets can be fixed to the quartz glass with an adhesive such as epoxy, with the result that there is no magnetic material in the electrode. This solution has been found unacceptable, because during prolonged usage the adhesive tends to harden and crack. The thermally cycling environment also causes degradation of the adhesive, with the result that the copper pulls free from the surface, and detunes the cavity. This failed electrode cannot be readily repaired when the clock apparatus is in an unmanned orbiting satellite. It is believed that all approaches for physically attaching a conductive layer suffer from essentially the same problems, and the lack of reliability of such an approach precludes its use in this application.
There therefore exists a need for a technique for preparing a highly conductive silver or copper electrode on a quartz glass substrate, without the presence of any magnetic material. Such a technique must be reproducible, and must produce high quality electrodes that are reliable in use over extended periods of time. The present invention fulfills this need, and further provides related advantages.